Continued: Ok, understand a bit more now.  Still not sure where the clone 
should be created but if the same renderer is called from more than one thread, 
as the first time startRender is called it sets the thread id for the 
rendererer, and thereafter cannot be called with a different thread id.  So 
issue could either be if startRender does not invoke super.startRender (to set 
thread id) or if called from two threads.
________________________________________
From: QGIS-Developer [[email protected]] on behalf of 
Chris Crook
Sent: 08 February 2018 08:45
To: [email protected]
Subject: [QGIS-Developer] QGIS3: Python QgsFeatureRenderer subclass problem

I am converting a python  plugin the implements a subclass of 
QgsFeatureRenderer.  At the moment the plugin is dying on the startRender 
function with the message (from the base class)

"ASSERT failure in QgsFeatureRenderer::renderFeature: \"renderFeature called in 
a different thread - use a cloned renderer instead\"

I can't see where I should be creating the cloned renderer for the rendering.  
I had a look at the QgsSingleSymbolRenderer C++ code and I can't see anything 
there, so I'm wondering if the python implementation in some way corrupts the 
multiprocessing implementation.  Can anyone offer any suggestions

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